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The interaction of KCl with tungsten single crystal surfaces
Surface Science 0 North-holland
94 (1980) 57-72 Publishing Company
THE INTERACTION
OF KC1 WITH TUNGSTEN SINGLE CRYSTAL
SURFACES
F. BONCZEK *, T. ENGEL ** and E. BAUER Physikalisches Institut der Technischen Universitri’t Clausthal, Leibnizstrasse 4, D-3392 Clausthal-Zellerfeld, and SFB 126, Giittingen-Clausthal, Germany Received
21 August
1979; accepted
for publication
10 December
1979
The interaction of KC1 vapor with W single crystal surfaces is studied with the aim of obtaining information on the adsorption of ionically bonded molecules. The atomically smoothest and roughest surfaces, (110) and (ill), are used. The adsorbate is characterized by LEED, thermal desorption spectroscopy (TDS) and by photoemission experiments which can be related to known work function data. It is found that the adsorbate is undissociated on W(110) and to a large extent also on W(lll) at room temperature and that no ordered structures are formed. Electron bombardment causes KC1 dissociation and Cl desorption which is complete with sufficiently high electron doses. Close-packed K monolayers can be formed this way.
1. Introduction
The study of the adsorption of heteronuclear diatomic molecules on single crystal surfaces has up to now been limited to a few covalently bonded molecules such as CO or NO. In order to obtain a better understanding of molecule-surface interactions it would be desirable to have some information on the adsorption of predominantly ionically bonded molecules such as HCl or other halides. This paper represents a first step in this direction. The halide choosen for this purpose is KCl, for several reasons: (1) because of its low vapor pressure it allows to study not only adsorption but also condensation, i.e. film growth, in a similar manner as it is being done increasingly with metal vapors on metal single crystal substrates (see ref. [I]). (2) Adsorbed and condensed KC1 layers promise to provide a better understanding of the dissociation of alkali halides upon electron bombardment which has been studied considerably using alkali halide single crystals [2-51. (3) By proper experimental procedures Cl adsorption layers can be produced
* Present address: Fachbereich Maschinenbau, Hochschule der Bundeswehr Hamburg, stenhofweg 85, D-2000 Hamburg 70, Germany. ** Present address: IBM Research Laboratory Zurich, CH-8803 Rtischlikon, Switzerland. 51
Hol-
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F. Bonczek
et al. /Interaction
of KC1 with tungsten
without introducing the highly reactive halogen gas directly into the UHV system which causes serious experimental problems. (4) Finally, the impinging KC1 flux and desorbed KC1 and K can be easily and accurately detected by surface ionization (for a review, see ref. [6]). Some work has been done in the past on the interaction between KC1 and tungsten. Silver and Witte [7] have studied the average changes occurring in KC1 layers on W upon heating, using field emission microscopy. Scheer et al. [8] investigated halogen adsorption on polycrystalline W by impinging an alkali halide beam on the surface at such high temperatures that decomposition occurred, but leading only to low halogen coverages. Datz and Taylor [9] and others [IO] studied surface ionization of KC1 on a variety of polycrystalline metal surfaces including W. The goal of this work is to study the adsorption, growth and stability of KC1 on individual W single crystal faces which differ significantly in their atomic roughness: the most densely packed (110) surface and the very rough (111) surface. The production of Cl layers on these surfaces and their characteristics will be dealth with in a subsequent paper [ 111.
2. Experimental The experiments were carried out in a stainless steel UHV system which after bakeout was capable of a base pressure of 1 X lo-lo Torr. The W(110) and W(111) substrates were prepared and cleaned using standard techniques. The surfaces were characterized with LEED and thermal desorption measurements. The thermal desorption spectra were recorded both with a quadrupole mass spectrometer and a tungsten wire surface ionization detector since, as will be shown below, the two methods give complementary information. Direct A# measurements (using the retarding field method) were attempted but not successful because of disturbances by electron stimulated desorption and photoemission as discussed later. KC1 was evaporated from a quartz tube which was heated with an external tungsten wire coil. The quartz tube together with an ionization detector used to monitor the effusion rate was surrounded by heat shields and equipped with a shutter to permit controlled KC1 exposures onto the substrates investigated.
3. Results 3.1. Surface ionization ofKC1 on W(110) and W(Il1) The Saha-Langmuir equation predicts [6] that the degree of ionization, species incident on the surface of a metal is given by
(Y,of a
F. Bonczek
et al. /Interaction
of KC7with tungsten
59
where Vi is the first ionization potential of the species and Q is the work function of the metal substrate. o+ and w. are partition function of the ion and neutral species respectively. For polycrystalline tungsten, Datz and Taylor [9] reported that the detection efficiency for KC1 and K at high temperatures is identical, implying that KC1 incident on the surface is at first dissociated and the K is subsequently surface ionized. Identical detection efficiencies for K and KI as well as KBr on polycrystalline tungsten have also been reported [IO]. It will therefore be assumed that the species which is surface ionized in this study is K, so that vi in eq. (1) is 4.34 eV. Of the low index tungsten planes, the (110) surface has the highest work function, with 5.25 eV being the most preferred value [ 121. Substitution of Vi and Qtlr0 into eq. (1) shows that Q: is practically one above room temperature, decreasing only very slowly with temperature. Therefore the (110) surface can be used as an absolute surface ionization detector to calibrate the polycrystalline detector (operated at 1400 K) in the KC1 oven. The experimentally observed ion current is shown in fig. 1 as a function of substrate temperature for KC1 incident on (110) and (111) surfaces. The high tempera-
+ r
100 o- . . . . .. .. . . ...oe.
. . . . * .* . .
.Q
. . . . . . .; . . . . b”‘;c;y--4;..0
. . . . . . . . . . . . .+
(110)
r
. . .. . . . . . . . .
0
+
+
g
75
I 1500
O0
1000 TEMPERATURE
L 2000
lK1-
Fig. 1. Degree of ionization, OL,as a function of temperature for W(ll0) and W(111): (+f, (X) this work; (0) data for pofycrystalline tungsten from ref. [9]; (0) calculated from eq. (I) with 9: 1o = 5.15 eV, W+/WO = 0.5; (0) calculated from cq. (1) with @111 = 4.45 eV, W+/WQ = 0.5.
60
F. Bonczek
et al. /Interaction
of KCl with tungsten
ture plateau in the ion current for W(110) has been assigned to the value of 01= 1 based on the prediction of eq. (1). With this assignment the maximum value of o for the (111) surface is approximately 0.45. Below 700 K for the W(110) and 800 K for the W(111) surfaces, Q drops sharply with decreasing temperature. This decrease is due to the buildup of K and Cl on the surface which leads to a decrease in r$ and therefore to a decrease of CY[6,13]. The high temperature part of the (Y curve for W(111) can be fitted approximately to eq. (1) if $~rrr = 4.37 eV is choosen instead of the preferred value of 4.45 [ 121 used in fig. 1. Possible causes for this difference are: (1) residual, slightly temperature dependent K adsorption which reduces the work function or/and (2) differing heats of desorption E for K+ and K which were assumed to be equal in the derivation of eq. (1). If the second cause would be solely responsible for the difference, E+ = E,, + 0.1 eV would be required which is compatable with other experiments [ 141; (3) the clean surface could really have a work function of 4.37 eV due to surface roughness. 3.2. Adsorption
of KC1 on W(ll0)
Fig. 2a shows thermal desorption curves taken with the quadrupole mass spectrometer for an exposure of 6.5 X 10 l4 KCl/cm* at a substrate temperature of 300K. (A W monolayer corresponds to 1.42 X 1015 atoms on the (110) surface.) Both K and Cl are seen to desorb near 700 K. An additional peak for K is seen at 1050 K which remains after switching off the ion source filament, showing that the desorbing species is K+. Fig. 2b shows a desorption spectrum obtained with the surface ionization detector for an exposure of 5.6 X 1014 KCl/cm2 at 300 K. A single sharp peak is observed near 700 K. Since the detection efficiency for K and KC1 incident on the ionization detector is identical [9], it is not possible to identify the desorbing species uniquely. However, no desorbing KC1 is detected with the quadrupole. This indicates either that KC1 is completely dissociated in the ion source or that K and Cl desorb separately as atoms at identical temperatures. These two possibilities can be distinguished in the following ways: (i) KC1 was evaporated directly into the ion source. No mass corresponding to KC1 was observed. Thus, the species desorbing at 700 K could be KCl, even if no such mass can be detected. (ii) Cl desorption spectra are taken from Cl adlayers (see ref. [ 1 l]), as well as from KC1 layers producing the desorption peak near 700 K. The relative detection efficiency for Cl as a function of the energy of the electrons in the quadrupole ionizer is shown in fig. 3 for the two cases. It is seen that the two cases are easily distinguishable and that at the electron energy of 100 eV normally used, the efficiency of Cl+ production from Cl is a factor of 15 higher than for Cl+ production from the species desorbing from a KC1 layer. The totally different dependences of the detection efficiency on electron energy seen in fig. 3 clearly show that the 700 K peak in fig. 2a is not due to atomic Cl but rather to molecular KCl.
61
F. Bonczek et al. /Interaction of KCI with tungsten I
MASS
I
IONIZATION DETECTOR
SPECTROMETER
1 I
:
__--c
/ 1’
Cl (351.2.5 _--
-_----
/\,
_.
300
_.A.-.
.
700
1’
K 1391 \._
b
1100 300 TEMPERATURE
700
1100
IKI -
Fig. 2. Thermal desorption curves after KC1 adsorption on the (110) surface at 300 K. (a) Desorption curves recorded with the quadrupole mass spectrometer: (- - -), ( -) ion source filament on; (- . -) ion source filament off; exposure 6.5 X 1014 KCl/cma; heating rate 35 K/s. (b) Desorption curves recorded with the ionization detector; exposure 5.6 X 1014 KCl/ cm2 ; heating rate 26 K/s.
Fig. 4 shows a series of desorption spectra, taken with the ionization detector, for 300 K adsorption on the (110) surface up to an exposure of 2.7 X 1014 KCI/ cm2. For small coverages, a broad peak between 800 and 1100 K is observed. At intermediate coverages, this peak saturates and the predominant KC1 peak at 700 K grows. At higher coverages, this peak grows further without an appreciable shift in temperature. Fig. 5 shows a plot of the area under the desorption peak as a function of the KC1 exposure up to an exposure of 5 X 10r5 KCl/cm2. The slope of this curve which is proportional to the sticking coefficient, s, is constant over the entire range, strongly suggesting that the sticking coefficient is unity. The results of an analysis of such KC1 desorption curves up to exposures of 5 X 1015 KCl/cm2 to obtain the desorption parameters E and v and their coverage dependence [ 15 ] are shown in fig. 6. It is seen that the activation energy for desorption, E, increases from 1 eV at low coverages up to approximately 2 eV at high coverages which agrees well with the bulk sublimation value of KC1 of 2.1 eV. Since a compensation law is observed between E and log v, only one curve for both quantities is plotted in fig. 6. The analysis is consistent with a first order desorption up to coverages of 3 monolayers. The physical process responsible for such an
62
F. Bonczek et al. [Interaction of KC1 with tungsten
25
50 ELECTRON
75 ENERGY
100
leVl-
Fig. 3. Relative sensitivity for Cl detection from Cl (ref. [ 111) and from KC1 layers as a function of the electron energy in the ion source of the quadrupole mass spectrometer.
unusual desorption behaviour is not clear at present. A completely different desorption behaviour, zero order desorption with the desorption curves showing a common leading edge, is observed for adsorption of KCl under vacuum conditions under which coadsorption of CO cannot be ruled out. Under these conditions it is likely that small three-dimensional KC1 crystals are formed from the very beginning of the exposure to KCl. The activation energy as estimated from the common leading edge is 1.35 eV which is lower than the heat of sublimation of bulk KCI. For KCl adsorption on a W(110) surface precovered with 6 X 1014 Cl/cm2, the broad high temperature peak observed in fig. 4a largely disappears as shown in fig. 4b. As a consequence the dependence of E and u on coverage is also somewhat different: A sharp rise in E and log v is observed for coverages below about 1 X 101” cm-a, after which E and log Y are constant at 2.4 eV and 18 respectively. This desorption behaviour is not understood either.
F. Bonczek et al. /Interaction of KCl with tungsten
63
-r
b
300
700
1100 TEMPERATURE
300 (KI
-
Fig. 4. Desorption curves registered with the ionization detector after exposure of the W(110) surface to KC1 at 300 K. (a) KC1 deposition onto clean surface; for the highest peak shown here the exposure is 2.7 X 10’4 KCl/cma; heating rate 26 K/s. (b) KC1 deposition onto surface covered with 6 X 1014 Cl atoms/cm2. Maximum exposure shown 1.5 X 1Or4 KCl/cm2 ; heating rate 22 K/s.
0
10
20 KCI
EXPOSURE
30 I x lO-“~rn~~l
LO -
Fig. 5. Area under the desorption curve as a function of KC1 exposure.
50
F. Bonczek et al. /‘Interactionof KC? with tungsten
64
2.0
1.5 I s z! w 1.0 -6
.5 0
10
20 KC1 COVERAGE
Fig. 6. Desorption coverage.
parameters
30 (~lO”~crn-~l
LO
50
-----v
E and log v for KC1 adsorbed
on W(110)
as a function
of the
The adsorption of KC1 on the (Ill) surface differs in a number of ways from adsorption on the (110) surface. Fig. 7 shows a series of desorption curves obtained with the ionization detector for exposures up to 8.5 X 1014 KCl/cm2. (A monolayer of W atoms on the (111) surface corresponds to 5.8 X 1014 atoms/cm*.) At low coverages a broad peak centered at 750 K grows without a shift in temperature until saturation is reached. Further adsorption of KC1 leads to the growth of a sharp peak at 600 K with a coverage-independent leading edge. The corresponding TD spectrum for Cl obtained with the quadrupole mass spectrometer is shown in fig. 8. In addition to the peaks at 600 and ‘750 K which will be designated (Yand 0 peaks, respectively, an additions Cl peak is seen at 1325 K, which will be designated the y peak. This peak appears to reach saturation before desorption from the low temperature peaks is observed. An identification of the desorbing species is clear for the 1325 K peak in which only Cl desorbs. A further analysis of the desorption products can be made under the assumption that the effective sticking coefficient for Cl is independent of coverage for the p and y peaks in analogy to the constant sticking coefficient for KC1 on W( 1 IO). This allows a determination of the relative sensitivity of the quadrupole for the Cl in the y and fl peaks since A’= C(F&
*F+&
where Ap and A, are the areas under the flash desorption
(2) curves, Fa and C;? are the
65
0 TEMPERATURE
fl<) -
Fig. 7. Desorption curves recorded with the ionizatiart detector as a function of temperature for KC1 adsorbed on W(111). The maximum exposure shown here is 8.5 X 1014 KCl/cmz. Heating rate: 62 K/s.
TEMPERATURE
IK 1-
Fig. 8. Desorption curves registered with the quadrupole (mass 35) after exposures of the W(11 1) substrate to (from bottom) Z-8,4.6,6.6, and 9.3 X 1014 KCl/cmz. Heating rate 73 K/s.
66
F. Bonczek
et al. /interaction
of KC’Z with tungsten
sensitivities of the quadrupole for Cl in the @and y peaks, N is the exposure of KC1 and C is a constant containing the effective sticking coefficient for CL With the above assumption, the ratio Fp]Fy is determined to be 1.4 + 0.3. This is equal to the correction factor (Ty/Tp)‘/*, which must be made for the different mean speeds of identical products desorbing at different temperatures T, and To. Since fig. 3 shows that the sensitivities for KC1 and Cl differ by a factor of 15 for the 100 eV electrons used here, the above result is only consistent with the /3 peak corresponding to dcsorption of atomic Cl rather than to the desorption of Cl in the form of KU. However, since a peak is observed in the same temperature range with the ionization detector, K and Cl must both desorb as atoms in the range corresponding to the 0 peak. The (x peak which desorbs at 600 K obeys zero order kinetics as is seen in the common leading edge of the desorption curves. An extension of the assumption of constant sticking coefficient to the coverage range in which the cr peak is observed leads to the conclusion that the cypeak consists of a mixture of KCl, K and Cl with KCI dominating. The activation energy for desorption associated with the ar peak is estimated from the leading edge to be 1.3 eV. No systematic analysis of the desorption parameters of the p peak has been made. The desorption of Cl in the y peak will be discussed in more detail in ref. [ 1 I]. Whereas the desorption curves for Cl are consistent with a constant sticking coefficient for Cl and show that the p and y peaks are due to atomic Cl and that the (Ypeak is due to both KC1 and Cl, coverages from desorption curves obtained with the ionization detector as shown in fig. 9 show a somewhat different behaviour. It is seen that the sticking coefficient which is proportional to the slope of the curve increases with coverage. This, however, is
Fig. 9. Area under desorption curves registered with the ionization detector for KC1 adsorption on W(ll1) as a function of the KC1 exposure.
67
F. Bonczek et al. /Interaction of KC1 with tungsten
the sticking coefficient for K and KC1 which may in fact differ from that for Cl. If, in analogy to the results on W(l IO), it can be assumed that the sticking coefficient for KC1 at high coverages approaches unity, the coverages of the species K, Cl and KC1 can be determined as a function of the KC1 exposure. The results are shown in fig. 10. It is seen that with the above assumption for coverages below 5 X 1014 KCl/ amounts of K and Cl are adsorbed. Above this coverage, cm2, nonstoic~ometric stoichiometric amounts of K and Cl are adsorbed with a sticking coefficient of unity. The saturation coverage for K in the 0 peak is 3.8 X 1014 atoms/cm2 and for Cl in the /!I and y peaks 8 X 1Or3 atoms/cm2 and 1.7 X 1014 atoms/cm2, respectively. Within the limits of error in absolute coverage the total number of adsorbed atoms is identical to the number of W atoms in the surface. 3.4. Electron stimulated
ofadsorbed KC1
desorption
Previous investigations of electron impact on alkali halides [3,5] have shown that surfaces cleaved in air and in vacuum give LEED patterns characteristic of the bulk and that rapid decomposition of the KC1 surface occurs under the influence of the electron beam [2-S]. No LEED superstructures were observed in the present study suggesting that KC1 forms an amorphous or polycrystalline layer under these deposition conditions. On the (110) surface, electron bombardment with the LEED gun led to the appearance of a ring about the (00) spot which upon prolonged bombardment developed into a well ordered hexagonal structure. A systematic study of the electron stimulated dissociation and desorption was carried out with a filament
I
@*‘f-Cl Y-Cl
0
0
2 KCI
I. EXPOSURE
ix
6 lO++cm-*i
8
1
I
I
I
I
I
Peak Peak
-
10
-
Fig. 10. Coverage of the individual species adsorbed on W(111) as a function of the KC1 exposure.
68
F. Bonczek et al. / fntemction
of KC1 with tungsten
formed in such a way as to allow uniform bombardInent of the surface. Thermai desorption curves obtained without and subsequent to electron bombardment of the KC1 adlayer are shown in fig, 11. Electron bombardment leads to the disappearance of the KC1 peak centered at 700 K. A broad K desorption peak between 300 K and 600 K is observed after electron bombardment. Heating of the substrate to 500 K results in the disappearance of the LEED pattern. A quantitative analysis of the time dependence of the Cl peak yields a cross section for electron induced desorption of 8 X 10-i 7 cm2 for Cl. In contrast to Cl, no significant amount of K is desarbed upon electron bombardment. This can best be seen from a primitive photoemission experiment and from the LEED pattern formed. Upon switch~g on the ion source filament, a photoemission current from the W(1 10) sample due to the incident light is observed. The dependence of this current on the exposure to KC1 prior to electron bombardment is shown in fig. 12. Also pfotted in fig. 12 are the data for A# versus coverage for K on W(liO> of Schmidt and Comer [16]. It is seen that the maximum in the photoemission current coincides with the minimum in the work function which is consistent with an effective sticking coefficient for K of unity. From this results it may be concluded that practically no K is desorbed by electron bombardment ~ The LEED pattern observed after electron bombardment of a KC1 adlayer with coverage >3.5 X lOi KCljcm2 on the (1 IO) surface is shown in figs. 13a and 13b,
I
L
K I’ E
TEMPERATURE
IKI -
Fig. 11. Desorption curves registered with the quadrupole mass spectrometer, of K and Cl from without electron bombardment; W(110) after an exposure of 6.5 X 1Ol4 KCl/cm2: ( --+ (- - -) after 3.8 mC/cmz; (- . -) after 7.3 mC/cmz.
F. Bonczek et al. f Interaction of KC1 with tungsten
“0
2
4 6 8 IO 12 EXPOSURE~~l~1~~~-2)
2
L
69
&
Fig. 12. Relative photoemission current and work function change A@(ref. [ 161) as a function of exposure to KC1 molecules (left) and to K atoms (right).
\ \ +KlmI!L l0011
t
t----0-+----* \
A
\
\
\
\
\
0
A0
0
‘\
\
\
‘* -[IT01
----
0
A 0
A
\
0
0
A
\
A
A 00
0
Fig. 13. (a) and (b) are LEED patterns observed after prolonged electron bombardment (a) electron energy 48 eV, (b) electron energy 84 eV. (c) Reciprocal lattice corresponding to the LEED pattern of (a). Full circles: W spots; triangles: K spots; open circles: double scattering spots of the innermost W and K spots.
70
F. Bonczek
a
et al. /interaction
of’KCl with tungsten
b
Fig. 14. (a) Structure in real space corresponding to the reciprocal lattice construction of fig. 13~. (b) The same structure rotated 30” with respect to the substrate. Open circles, W atoms; shaded circles, K atoms.
and the reciprocal lattice with the unit mesh chosen is shown in fig. 13~. The corresponding structure in real space is shown in fig. 14a. It corresponds to a close packed K layer with a nearest neighbour spacing of 4.63 A which is equal to that found in bulk potassium. A second LEED pattern which is rotated 30” with respect to that in figs. 13a and 13b was also observed. This corresponds in real space to the structure shown in fig. 14b which appears to be somewhat less stable than that in fig. 14a. Both structures correspond to a coverage of 5.4 X 1Or4 K/cm2. Upon electron bombardment of the KC1 covered W(111) surface, all the Cl is desorbed. The cross section for Cl desorption is 8 X lo-18 cm2 and the maximum K coverage is 5.8 X 1014 atoms/cm* which corresponds to one monolayer on the (111) plane. Only a (1 X 1) pattern is observed with LEED which is presumably due to a 3.5% compression of the close packed structure observed on W(110) to one with a spacing of 4.477 A which is then identical to the tungsten spacing in the (111) surface.
4. Discussion The principal difference in the adsorption - desorption behaviour of KC1 on W( 111) and W( 110) is the degree to which decomposition of the adlayer occurs. On the (110) surface, no decomposition takes place without electron bombardment and the adsorption-desorption behaviour is that of KC1 alone. The sticking coefficient for KC1 is unity for coverages up to at least three monolayers. Since no LEED superstructures are observed and the sticking coefficient is independent of coverage, no direct information on the growth grocess is obtained. This can also not be
F. Bonczek
et al. /Interaction
of KCl with tungsten
71
obtained from the TDS studies because the unusual dependence of the desorption parameters upon coverage, such as shown in fig. 6, does not allow any conclusions concerning the structure of the layer before desorption. Simple three-dimensional KC1 crystallite formation seems, however, unlikely, except on contaminated surfaces. For the (111) surface, fig. 9 shows that the sticking coefficient rises with the KC1 coverage which is to be expected if critical nuclei must be formed before further adlayer growth takes place. The zero order desorption kinetics observed at high KC1 coverages shows that on the (111) surface KC1 tends more to form threedimensional nuclei than on the (110) surface. However, the thermal desorption curves of figs. 7 and 8 show that KC1 is desorbed as KC1 only at high coverages, whereas thermal desorption from a surface covered with less than a monolayer of KC1 results in decomposition and subsequent desorption of K and Cl. The question of whether the KC1 is dissociated on the (111) surface at 300 K can only be partially answered. Experiments with prolonged heating of an adlayer deposited at 300 K at temperatures below 600 K before thermal desorption, yielded more Cl in the -r peak than was obtained after adsorption at 300 K followed immediately by thermal desorption. This suggests that at least partial conversion of the adlayer takes place upon heating, and that the 300 K KC1 adlayer on the (111) surface is largely undissociated. The differences in the rates of decomposition of KC1 on the two tungsten orientations may be correlated with the more open structure of the (111) surface in which a KC1 molecule can be in intimate contact with several tungsten atoms in different layers as contrasted with the nearly close packed (110) surface for which such a close contact is not possible. A more detailed comparison of the decomposition rates of KC1 on the two surfaces will be made in connection with the completely different processes occurring during adsorption at elevated temperatures in ref. [ 111.
References [l] E. Bauer, J. Physique 38 (1977) C4-146. [2] H. Steffen, MS. thesis, Clausthal(l968); H. Steffen, R. Niedermayer and H. Mayer, Thin Solid Films 1 (1969) 223. [3] T.N. Rhodin, P.W. Palmberg and C.J. Todd, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis, R.D. Gretz and L. Jaffee (McGraw-Hill, New York, 1969) p. 504. [4] A. Green, E. Bauer and J. Dancy, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis, R.D. Gretz and L. Jaffee (McGraw-Hill, New York, 1969) p. 493. [5] T.E. Gallon, LG. Higgenbotham, M. Prutton and H. Tokutaka, Surface Sci. 21 (1970) 224; 233; 241. [6] N.I. Ionov, in: Progress in Surface Science, Vol. 1, Part 3, Ed. S.G. Davison (Pergamon, Oxford, 1972). [7] M. Silver and R. Witte, J. Chem. Phys. 38 (1963) 872. [ 81 M.D. Scheer, R. Klein and J.D. McKinley, Surface Sci. 30 (1972) 251. [9] S. Datz and E.H. Taylor, J. Chem. Phys. 25 (1956) 395.
F. Bonczek et al. /Interaction
12
[lo]
See ref. [6], p. 302. T. Engel, E. Bauer, to be published. H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729. E.F. Chaikovskii, G.M. Pyatigorskii and Yu.F. Derkach, Bull. Acad. Sci. USSR, Phys. Ser. 35 (1971) 342. F. Bonczek, M.S. thesis, Clausthal 1975, p. 127. E. Bauer, F. Bonczek, H. Poppa and G. Todd, Surface Sci. 53 (1975) 87. L.D. Schmidt and R. Gomer, J. Chem. Phys. 45 (1966) 1605.
[ 111 F. Bonczek, [12]
[ 131 [14] [ 151 [16]
of KC1 with tungsten
94 (1980) 57-72 Publishing Company
THE INTERACTION
OF KC1 WITH TUNGSTEN SINGLE CRYSTAL
SURFACES
F. BONCZEK *, T. ENGEL ** and E. BAUER Physikalisches Institut der Technischen Universitri’t Clausthal, Leibnizstrasse 4, D-3392 Clausthal-Zellerfeld, and SFB 126, Giittingen-Clausthal, Germany Received
21 August
1979; accepted
for publication
10 December
1979
The interaction of KC1 vapor with W single crystal surfaces is studied with the aim of obtaining information on the adsorption of ionically bonded molecules. The atomically smoothest and roughest surfaces, (110) and (ill), are used. The adsorbate is characterized by LEED, thermal desorption spectroscopy (TDS) and by photoemission experiments which can be related to known work function data. It is found that the adsorbate is undissociated on W(110) and to a large extent also on W(lll) at room temperature and that no ordered structures are formed. Electron bombardment causes KC1 dissociation and Cl desorption which is complete with sufficiently high electron doses. Close-packed K monolayers can be formed this way.
1. Introduction
The study of the adsorption of heteronuclear diatomic molecules on single crystal surfaces has up to now been limited to a few covalently bonded molecules such as CO or NO. In order to obtain a better understanding of molecule-surface interactions it would be desirable to have some information on the adsorption of predominantly ionically bonded molecules such as HCl or other halides. This paper represents a first step in this direction. The halide choosen for this purpose is KCl, for several reasons: (1) because of its low vapor pressure it allows to study not only adsorption but also condensation, i.e. film growth, in a similar manner as it is being done increasingly with metal vapors on metal single crystal substrates (see ref. [I]). (2) Adsorbed and condensed KC1 layers promise to provide a better understanding of the dissociation of alkali halides upon electron bombardment which has been studied considerably using alkali halide single crystals [2-51. (3) By proper experimental procedures Cl adsorption layers can be produced
* Present address: Fachbereich Maschinenbau, Hochschule der Bundeswehr Hamburg, stenhofweg 85, D-2000 Hamburg 70, Germany. ** Present address: IBM Research Laboratory Zurich, CH-8803 Rtischlikon, Switzerland. 51
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F. Bonczek
et al. /Interaction
of KC1 with tungsten
without introducing the highly reactive halogen gas directly into the UHV system which causes serious experimental problems. (4) Finally, the impinging KC1 flux and desorbed KC1 and K can be easily and accurately detected by surface ionization (for a review, see ref. [6]). Some work has been done in the past on the interaction between KC1 and tungsten. Silver and Witte [7] have studied the average changes occurring in KC1 layers on W upon heating, using field emission microscopy. Scheer et al. [8] investigated halogen adsorption on polycrystalline W by impinging an alkali halide beam on the surface at such high temperatures that decomposition occurred, but leading only to low halogen coverages. Datz and Taylor [9] and others [IO] studied surface ionization of KC1 on a variety of polycrystalline metal surfaces including W. The goal of this work is to study the adsorption, growth and stability of KC1 on individual W single crystal faces which differ significantly in their atomic roughness: the most densely packed (110) surface and the very rough (111) surface. The production of Cl layers on these surfaces and their characteristics will be dealth with in a subsequent paper [ 111.
2. Experimental The experiments were carried out in a stainless steel UHV system which after bakeout was capable of a base pressure of 1 X lo-lo Torr. The W(110) and W(111) substrates were prepared and cleaned using standard techniques. The surfaces were characterized with LEED and thermal desorption measurements. The thermal desorption spectra were recorded both with a quadrupole mass spectrometer and a tungsten wire surface ionization detector since, as will be shown below, the two methods give complementary information. Direct A# measurements (using the retarding field method) were attempted but not successful because of disturbances by electron stimulated desorption and photoemission as discussed later. KC1 was evaporated from a quartz tube which was heated with an external tungsten wire coil. The quartz tube together with an ionization detector used to monitor the effusion rate was surrounded by heat shields and equipped with a shutter to permit controlled KC1 exposures onto the substrates investigated.
3. Results 3.1. Surface ionization ofKC1 on W(110) and W(Il1) The Saha-Langmuir equation predicts [6] that the degree of ionization, species incident on the surface of a metal is given by
(Y,of a
F. Bonczek
et al. /Interaction
of KC7with tungsten
59
where Vi is the first ionization potential of the species and Q is the work function of the metal substrate. o+ and w. are partition function of the ion and neutral species respectively. For polycrystalline tungsten, Datz and Taylor [9] reported that the detection efficiency for KC1 and K at high temperatures is identical, implying that KC1 incident on the surface is at first dissociated and the K is subsequently surface ionized. Identical detection efficiencies for K and KI as well as KBr on polycrystalline tungsten have also been reported [IO]. It will therefore be assumed that the species which is surface ionized in this study is K, so that vi in eq. (1) is 4.34 eV. Of the low index tungsten planes, the (110) surface has the highest work function, with 5.25 eV being the most preferred value [ 121. Substitution of Vi and Qtlr0 into eq. (1) shows that Q: is practically one above room temperature, decreasing only very slowly with temperature. Therefore the (110) surface can be used as an absolute surface ionization detector to calibrate the polycrystalline detector (operated at 1400 K) in the KC1 oven. The experimentally observed ion current is shown in fig. 1 as a function of substrate temperature for KC1 incident on (110) and (111) surfaces. The high tempera-
+ r
100 o- . . . . .. .. . . ...oe.
. . . . * .* . .
.Q
. . . . . . .; . . . . b”‘;c;y--4;..0
. . . . . . . . . . . . .+
(110)
r
. . .. . . . . . . . .
0
+
+
g
75
I 1500
O0
1000 TEMPERATURE
L 2000
lK1-
Fig. 1. Degree of ionization, OL,as a function of temperature for W(ll0) and W(111): (+f, (X) this work; (0) data for pofycrystalline tungsten from ref. [9]; (0) calculated from eq. (I) with 9: 1o = 5.15 eV, W+/WO = 0.5; (0) calculated from cq. (1) with @111 = 4.45 eV, W+/WQ = 0.5.
60
F. Bonczek
et al. /Interaction
of KCl with tungsten
ture plateau in the ion current for W(110) has been assigned to the value of 01= 1 based on the prediction of eq. (1). With this assignment the maximum value of o for the (111) surface is approximately 0.45. Below 700 K for the W(110) and 800 K for the W(111) surfaces, Q drops sharply with decreasing temperature. This decrease is due to the buildup of K and Cl on the surface which leads to a decrease in r$ and therefore to a decrease of CY[6,13]. The high temperature part of the (Y curve for W(111) can be fitted approximately to eq. (1) if $~rrr = 4.37 eV is choosen instead of the preferred value of 4.45 [ 121 used in fig. 1. Possible causes for this difference are: (1) residual, slightly temperature dependent K adsorption which reduces the work function or/and (2) differing heats of desorption E for K+ and K which were assumed to be equal in the derivation of eq. (1). If the second cause would be solely responsible for the difference, E+ = E,, + 0.1 eV would be required which is compatable with other experiments [ 141; (3) the clean surface could really have a work function of 4.37 eV due to surface roughness. 3.2. Adsorption
of KC1 on W(ll0)
Fig. 2a shows thermal desorption curves taken with the quadrupole mass spectrometer for an exposure of 6.5 X 10 l4 KCl/cm* at a substrate temperature of 300K. (A W monolayer corresponds to 1.42 X 1015 atoms on the (110) surface.) Both K and Cl are seen to desorb near 700 K. An additional peak for K is seen at 1050 K which remains after switching off the ion source filament, showing that the desorbing species is K+. Fig. 2b shows a desorption spectrum obtained with the surface ionization detector for an exposure of 5.6 X 1014 KCl/cm2 at 300 K. A single sharp peak is observed near 700 K. Since the detection efficiency for K and KC1 incident on the ionization detector is identical [9], it is not possible to identify the desorbing species uniquely. However, no desorbing KC1 is detected with the quadrupole. This indicates either that KC1 is completely dissociated in the ion source or that K and Cl desorb separately as atoms at identical temperatures. These two possibilities can be distinguished in the following ways: (i) KC1 was evaporated directly into the ion source. No mass corresponding to KC1 was observed. Thus, the species desorbing at 700 K could be KCl, even if no such mass can be detected. (ii) Cl desorption spectra are taken from Cl adlayers (see ref. [ 1 l]), as well as from KC1 layers producing the desorption peak near 700 K. The relative detection efficiency for Cl as a function of the energy of the electrons in the quadrupole ionizer is shown in fig. 3 for the two cases. It is seen that the two cases are easily distinguishable and that at the electron energy of 100 eV normally used, the efficiency of Cl+ production from Cl is a factor of 15 higher than for Cl+ production from the species desorbing from a KC1 layer. The totally different dependences of the detection efficiency on electron energy seen in fig. 3 clearly show that the 700 K peak in fig. 2a is not due to atomic Cl but rather to molecular KCl.
61
F. Bonczek et al. /Interaction of KCI with tungsten I
MASS
I
IONIZATION DETECTOR
SPECTROMETER
1 I
:
__--c
/ 1’
Cl (351.2.5 _--
-_----
/\,
_.
300
_.A.-.
.
700
1’
K 1391 \._
b
1100 300 TEMPERATURE
700
1100
IKI -
Fig. 2. Thermal desorption curves after KC1 adsorption on the (110) surface at 300 K. (a) Desorption curves recorded with the quadrupole mass spectrometer: (- - -), ( -) ion source filament on; (- . -) ion source filament off; exposure 6.5 X 1014 KCl/cma; heating rate 35 K/s. (b) Desorption curves recorded with the ionization detector; exposure 5.6 X 1014 KCl/ cm2 ; heating rate 26 K/s.
Fig. 4 shows a series of desorption spectra, taken with the ionization detector, for 300 K adsorption on the (110) surface up to an exposure of 2.7 X 1014 KCI/ cm2. For small coverages, a broad peak between 800 and 1100 K is observed. At intermediate coverages, this peak saturates and the predominant KC1 peak at 700 K grows. At higher coverages, this peak grows further without an appreciable shift in temperature. Fig. 5 shows a plot of the area under the desorption peak as a function of the KC1 exposure up to an exposure of 5 X 10r5 KCl/cm2. The slope of this curve which is proportional to the sticking coefficient, s, is constant over the entire range, strongly suggesting that the sticking coefficient is unity. The results of an analysis of such KC1 desorption curves up to exposures of 5 X 1015 KCl/cm2 to obtain the desorption parameters E and v and their coverage dependence [ 15 ] are shown in fig. 6. It is seen that the activation energy for desorption, E, increases from 1 eV at low coverages up to approximately 2 eV at high coverages which agrees well with the bulk sublimation value of KC1 of 2.1 eV. Since a compensation law is observed between E and log v, only one curve for both quantities is plotted in fig. 6. The analysis is consistent with a first order desorption up to coverages of 3 monolayers. The physical process responsible for such an
62
F. Bonczek et al. [Interaction of KC1 with tungsten
25
50 ELECTRON
75 ENERGY
100
leVl-
Fig. 3. Relative sensitivity for Cl detection from Cl (ref. [ 111) and from KC1 layers as a function of the electron energy in the ion source of the quadrupole mass spectrometer.
unusual desorption behaviour is not clear at present. A completely different desorption behaviour, zero order desorption with the desorption curves showing a common leading edge, is observed for adsorption of KCl under vacuum conditions under which coadsorption of CO cannot be ruled out. Under these conditions it is likely that small three-dimensional KC1 crystals are formed from the very beginning of the exposure to KCl. The activation energy as estimated from the common leading edge is 1.35 eV which is lower than the heat of sublimation of bulk KCI. For KCl adsorption on a W(110) surface precovered with 6 X 1014 Cl/cm2, the broad high temperature peak observed in fig. 4a largely disappears as shown in fig. 4b. As a consequence the dependence of E and u on coverage is also somewhat different: A sharp rise in E and log v is observed for coverages below about 1 X 101” cm-a, after which E and log Y are constant at 2.4 eV and 18 respectively. This desorption behaviour is not understood either.
F. Bonczek et al. /Interaction of KCl with tungsten
63
-r
b
300
700
1100 TEMPERATURE
300 (KI
-
Fig. 4. Desorption curves registered with the ionization detector after exposure of the W(110) surface to KC1 at 300 K. (a) KC1 deposition onto clean surface; for the highest peak shown here the exposure is 2.7 X 10’4 KCl/cma; heating rate 26 K/s. (b) KC1 deposition onto surface covered with 6 X 1014 Cl atoms/cm2. Maximum exposure shown 1.5 X 1Or4 KCl/cm2 ; heating rate 22 K/s.
0
10
20 KCI
EXPOSURE
30 I x lO-“~rn~~l
LO -
Fig. 5. Area under the desorption curve as a function of KC1 exposure.
50
F. Bonczek et al. /‘Interactionof KC? with tungsten
64
2.0
1.5 I s z! w 1.0 -6
.5 0
10
20 KC1 COVERAGE
Fig. 6. Desorption coverage.
parameters
30 (~lO”~crn-~l
LO
50
-----v
E and log v for KC1 adsorbed
on W(110)
as a function
of the
The adsorption of KC1 on the (Ill) surface differs in a number of ways from adsorption on the (110) surface. Fig. 7 shows a series of desorption curves obtained with the ionization detector for exposures up to 8.5 X 1014 KCl/cm2. (A monolayer of W atoms on the (111) surface corresponds to 5.8 X 1014 atoms/cm*.) At low coverages a broad peak centered at 750 K grows without a shift in temperature until saturation is reached. Further adsorption of KC1 leads to the growth of a sharp peak at 600 K with a coverage-independent leading edge. The corresponding TD spectrum for Cl obtained with the quadrupole mass spectrometer is shown in fig. 8. In addition to the peaks at 600 and ‘750 K which will be designated (Yand 0 peaks, respectively, an additions Cl peak is seen at 1325 K, which will be designated the y peak. This peak appears to reach saturation before desorption from the low temperature peaks is observed. An identification of the desorbing species is clear for the 1325 K peak in which only Cl desorbs. A further analysis of the desorption products can be made under the assumption that the effective sticking coefficient for Cl is independent of coverage for the p and y peaks in analogy to the constant sticking coefficient for KC1 on W( 1 IO). This allows a determination of the relative sensitivity of the quadrupole for the Cl in the y and fl peaks since A’= C(F&
*F+&
where Ap and A, are the areas under the flash desorption
(2) curves, Fa and C;? are the
65
0 TEMPERATURE
fl<) -
Fig. 7. Desorption curves recorded with the ionizatiart detector as a function of temperature for KC1 adsorbed on W(111). The maximum exposure shown here is 8.5 X 1014 KCl/cmz. Heating rate: 62 K/s.
TEMPERATURE
IK 1-
Fig. 8. Desorption curves registered with the quadrupole (mass 35) after exposures of the W(11 1) substrate to (from bottom) Z-8,4.6,6.6, and 9.3 X 1014 KCl/cmz. Heating rate 73 K/s.
66
F. Bonczek
et al. /interaction
of KC’Z with tungsten
sensitivities of the quadrupole for Cl in the @and y peaks, N is the exposure of KC1 and C is a constant containing the effective sticking coefficient for CL With the above assumption, the ratio Fp]Fy is determined to be 1.4 + 0.3. This is equal to the correction factor (Ty/Tp)‘/*, which must be made for the different mean speeds of identical products desorbing at different temperatures T, and To. Since fig. 3 shows that the sensitivities for KC1 and Cl differ by a factor of 15 for the 100 eV electrons used here, the above result is only consistent with the /3 peak corresponding to dcsorption of atomic Cl rather than to the desorption of Cl in the form of KU. However, since a peak is observed in the same temperature range with the ionization detector, K and Cl must both desorb as atoms in the range corresponding to the 0 peak. The (x peak which desorbs at 600 K obeys zero order kinetics as is seen in the common leading edge of the desorption curves. An extension of the assumption of constant sticking coefficient to the coverage range in which the cr peak is observed leads to the conclusion that the cypeak consists of a mixture of KCl, K and Cl with KCI dominating. The activation energy for desorption associated with the ar peak is estimated from the leading edge to be 1.3 eV. No systematic analysis of the desorption parameters of the p peak has been made. The desorption of Cl in the y peak will be discussed in more detail in ref. [ 1 I]. Whereas the desorption curves for Cl are consistent with a constant sticking coefficient for Cl and show that the p and y peaks are due to atomic Cl and that the (Ypeak is due to both KC1 and Cl, coverages from desorption curves obtained with the ionization detector as shown in fig. 9 show a somewhat different behaviour. It is seen that the sticking coefficient which is proportional to the slope of the curve increases with coverage. This, however, is
Fig. 9. Area under desorption curves registered with the ionization detector for KC1 adsorption on W(ll1) as a function of the KC1 exposure.
67
F. Bonczek et al. /Interaction of KC1 with tungsten
the sticking coefficient for K and KC1 which may in fact differ from that for Cl. If, in analogy to the results on W(l IO), it can be assumed that the sticking coefficient for KC1 at high coverages approaches unity, the coverages of the species K, Cl and KC1 can be determined as a function of the KC1 exposure. The results are shown in fig. 10. It is seen that with the above assumption for coverages below 5 X 1014 KCl/ amounts of K and Cl are adsorbed. Above this coverage, cm2, nonstoic~ometric stoichiometric amounts of K and Cl are adsorbed with a sticking coefficient of unity. The saturation coverage for K in the 0 peak is 3.8 X 1014 atoms/cm2 and for Cl in the /!I and y peaks 8 X 1Or3 atoms/cm2 and 1.7 X 1014 atoms/cm2, respectively. Within the limits of error in absolute coverage the total number of adsorbed atoms is identical to the number of W atoms in the surface. 3.4. Electron stimulated
ofadsorbed KC1
desorption
Previous investigations of electron impact on alkali halides [3,5] have shown that surfaces cleaved in air and in vacuum give LEED patterns characteristic of the bulk and that rapid decomposition of the KC1 surface occurs under the influence of the electron beam [2-S]. No LEED superstructures were observed in the present study suggesting that KC1 forms an amorphous or polycrystalline layer under these deposition conditions. On the (110) surface, electron bombardment with the LEED gun led to the appearance of a ring about the (00) spot which upon prolonged bombardment developed into a well ordered hexagonal structure. A systematic study of the electron stimulated dissociation and desorption was carried out with a filament
I
@*‘f-Cl Y-Cl
0
0
2 KCI
I. EXPOSURE
ix
6 lO++cm-*i
8
1
I
I
I
I
I
Peak Peak
-
10
-
Fig. 10. Coverage of the individual species adsorbed on W(111) as a function of the KC1 exposure.
68
F. Bonczek et al. / fntemction
of KC1 with tungsten
formed in such a way as to allow uniform bombardInent of the surface. Thermai desorption curves obtained without and subsequent to electron bombardment of the KC1 adlayer are shown in fig, 11. Electron bombardment leads to the disappearance of the KC1 peak centered at 700 K. A broad K desorption peak between 300 K and 600 K is observed after electron bombardment. Heating of the substrate to 500 K results in the disappearance of the LEED pattern. A quantitative analysis of the time dependence of the Cl peak yields a cross section for electron induced desorption of 8 X 10-i 7 cm2 for Cl. In contrast to Cl, no significant amount of K is desarbed upon electron bombardment. This can best be seen from a primitive photoemission experiment and from the LEED pattern formed. Upon switch~g on the ion source filament, a photoemission current from the W(1 10) sample due to the incident light is observed. The dependence of this current on the exposure to KC1 prior to electron bombardment is shown in fig. 12. Also pfotted in fig. 12 are the data for A# versus coverage for K on W(liO> of Schmidt and Comer [16]. It is seen that the maximum in the photoemission current coincides with the minimum in the work function which is consistent with an effective sticking coefficient for K of unity. From this results it may be concluded that practically no K is desorbed by electron bombardment ~ The LEED pattern observed after electron bombardment of a KC1 adlayer with coverage >3.5 X lOi KCljcm2 on the (1 IO) surface is shown in figs. 13a and 13b,
I
L
K I’ E
TEMPERATURE
IKI -
Fig. 11. Desorption curves registered with the quadrupole mass spectrometer, of K and Cl from without electron bombardment; W(110) after an exposure of 6.5 X 1Ol4 KCl/cm2: ( --+ (- - -) after 3.8 mC/cmz; (- . -) after 7.3 mC/cmz.
F. Bonczek et al. f Interaction of KC1 with tungsten
“0
2
4 6 8 IO 12 EXPOSURE~~l~1~~~-2)
2
L
69
&
Fig. 12. Relative photoemission current and work function change A@(ref. [ 161) as a function of exposure to KC1 molecules (left) and to K atoms (right).
\ \ +KlmI!L l0011
t
t----0-+----* \
A
\
\
\
\
\
0
A0
0
‘\
\
\
‘* -[IT01
----
0
A 0
A
\
0
0
A
\
A
A 00
0
Fig. 13. (a) and (b) are LEED patterns observed after prolonged electron bombardment (a) electron energy 48 eV, (b) electron energy 84 eV. (c) Reciprocal lattice corresponding to the LEED pattern of (a). Full circles: W spots; triangles: K spots; open circles: double scattering spots of the innermost W and K spots.
70
F. Bonczek
a
et al. /interaction
of’KCl with tungsten
b
Fig. 14. (a) Structure in real space corresponding to the reciprocal lattice construction of fig. 13~. (b) The same structure rotated 30” with respect to the substrate. Open circles, W atoms; shaded circles, K atoms.
and the reciprocal lattice with the unit mesh chosen is shown in fig. 13~. The corresponding structure in real space is shown in fig. 14a. It corresponds to a close packed K layer with a nearest neighbour spacing of 4.63 A which is equal to that found in bulk potassium. A second LEED pattern which is rotated 30” with respect to that in figs. 13a and 13b was also observed. This corresponds in real space to the structure shown in fig. 14b which appears to be somewhat less stable than that in fig. 14a. Both structures correspond to a coverage of 5.4 X 1Or4 K/cm2. Upon electron bombardment of the KC1 covered W(111) surface, all the Cl is desorbed. The cross section for Cl desorption is 8 X lo-18 cm2 and the maximum K coverage is 5.8 X 1014 atoms/cm* which corresponds to one monolayer on the (111) plane. Only a (1 X 1) pattern is observed with LEED which is presumably due to a 3.5% compression of the close packed structure observed on W(110) to one with a spacing of 4.477 A which is then identical to the tungsten spacing in the (111) surface.
4. Discussion The principal difference in the adsorption - desorption behaviour of KC1 on W( 111) and W( 110) is the degree to which decomposition of the adlayer occurs. On the (110) surface, no decomposition takes place without electron bombardment and the adsorption-desorption behaviour is that of KC1 alone. The sticking coefficient for KC1 is unity for coverages up to at least three monolayers. Since no LEED superstructures are observed and the sticking coefficient is independent of coverage, no direct information on the growth grocess is obtained. This can also not be
F. Bonczek
et al. /Interaction
of KCl with tungsten
71
obtained from the TDS studies because the unusual dependence of the desorption parameters upon coverage, such as shown in fig. 6, does not allow any conclusions concerning the structure of the layer before desorption. Simple three-dimensional KC1 crystallite formation seems, however, unlikely, except on contaminated surfaces. For the (111) surface, fig. 9 shows that the sticking coefficient rises with the KC1 coverage which is to be expected if critical nuclei must be formed before further adlayer growth takes place. The zero order desorption kinetics observed at high KC1 coverages shows that on the (111) surface KC1 tends more to form threedimensional nuclei than on the (110) surface. However, the thermal desorption curves of figs. 7 and 8 show that KC1 is desorbed as KC1 only at high coverages, whereas thermal desorption from a surface covered with less than a monolayer of KC1 results in decomposition and subsequent desorption of K and Cl. The question of whether the KC1 is dissociated on the (111) surface at 300 K can only be partially answered. Experiments with prolonged heating of an adlayer deposited at 300 K at temperatures below 600 K before thermal desorption, yielded more Cl in the -r peak than was obtained after adsorption at 300 K followed immediately by thermal desorption. This suggests that at least partial conversion of the adlayer takes place upon heating, and that the 300 K KC1 adlayer on the (111) surface is largely undissociated. The differences in the rates of decomposition of KC1 on the two tungsten orientations may be correlated with the more open structure of the (111) surface in which a KC1 molecule can be in intimate contact with several tungsten atoms in different layers as contrasted with the nearly close packed (110) surface for which such a close contact is not possible. A more detailed comparison of the decomposition rates of KC1 on the two surfaces will be made in connection with the completely different processes occurring during adsorption at elevated temperatures in ref. [ 111.
References [l] E. Bauer, J. Physique 38 (1977) C4-146. [2] H. Steffen, MS. thesis, Clausthal(l968); H. Steffen, R. Niedermayer and H. Mayer, Thin Solid Films 1 (1969) 223. [3] T.N. Rhodin, P.W. Palmberg and C.J. Todd, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis, R.D. Gretz and L. Jaffee (McGraw-Hill, New York, 1969) p. 504. [4] A. Green, E. Bauer and J. Dancy, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis, R.D. Gretz and L. Jaffee (McGraw-Hill, New York, 1969) p. 493. [5] T.E. Gallon, LG. Higgenbotham, M. Prutton and H. Tokutaka, Surface Sci. 21 (1970) 224; 233; 241. [6] N.I. Ionov, in: Progress in Surface Science, Vol. 1, Part 3, Ed. S.G. Davison (Pergamon, Oxford, 1972). [7] M. Silver and R. Witte, J. Chem. Phys. 38 (1963) 872. [ 81 M.D. Scheer, R. Klein and J.D. McKinley, Surface Sci. 30 (1972) 251. [9] S. Datz and E.H. Taylor, J. Chem. Phys. 25 (1956) 395.
F. Bonczek et al. /Interaction
12
[lo]
See ref. [6], p. 302. T. Engel, E. Bauer, to be published. H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729. E.F. Chaikovskii, G.M. Pyatigorskii and Yu.F. Derkach, Bull. Acad. Sci. USSR, Phys. Ser. 35 (1971) 342. F. Bonczek, M.S. thesis, Clausthal 1975, p. 127. E. Bauer, F. Bonczek, H. Poppa and G. Todd, Surface Sci. 53 (1975) 87. L.D. Schmidt and R. Gomer, J. Chem. Phys. 45 (1966) 1605.
[ 111 F. Bonczek, [12]
[ 131 [14] [ 151 [16]
of KC1 with tungsten